The present invention relates to a method and apparatus for selectively cooling areas of glass sheets during heating in the lehr in order to minimize undesirable sagging of portions of the glass sheet, particularly top edge portions bearing dark banding.
When it is desired to give a glass sheet a nonplanar shape, for example during the manufacture of an automobile glazing, the planar sheet is brought to a temperature, and deformation is caused either by stressing the malleable lass to assume a rigid shape, or by positioning it horizontally and allowing it to become deformed under its own weight. Once the desired shape is obtained, a controlled cooling of the glass is carried out. The most recently employed cooling processes are tempering and annealing.
The first, which generally comprises energetically blowing on the glass, provides it, once cooled, with a prestressed state which increases its resistance to bending and thermal shock. This process is used typically for manufacturing of glazings which are to equip the sides or the rear of automobiles.
In annealing, on the other hand, the relaxation of stresses, is permitted by cooling the glass in a very gradual manner. In this case, an annealed glass is obtained whose stress level is slight in the direction of the thickness of the glass, which optionally allows the finished product to be cut and which, in the case of an automobile glazing, avoids explosive breakage in case by an impact of fine gravel. This technique is therefore particularly adapted to the manufacturing of windshields.
Also known is a technique where two sheets of superimposed glass which are simultaneously heated, bent and cooled are then assembled in pairs with a plastic sheet inserted therebetween.
In addition to tempering and annealing, it is known to blow cooler air on the periphery of glass sheets at the beginning of the cooling process to create in this peripheral zone a slight compression prestressing which limits the risks of later breakage. To carry out this blowing, one system uses blowing hoods which have a truncated pyramid shape. The air is guided by the walls of the hood and then escapes in a free passage between the edge of the hood and the glass. In the center, the air circulation is much slower: the heat exchange is therefore increased at the periphery.
In another system, instead of guiding the air only in a hood in the shape of a flared duct, a pyramidal deflector is put at the end of the hood which, while covering the central part of the glass sheet, allows the air to reach only its periphery. See, Lecourt et al U.S. Pat. No. 4,871,385. The pyramidal shape gives a precise direction to the air currents. To adapt the blowing to the different glass sheets, the process of Lecourt et al provides that, by adjusting the distance between the deflector and the hot glass sheet, the periphery of the glass is blown on preferentially to a more or less large central zone.
While Lecourt et al provides a system for prestressing the entire periphery of a glass sheet to varying degrees of width, by definition the Lecourt et al process is one that occurs at the beginning of the cooling process. It does not address the sagging problems which may occur during heating in the lehr, particularly that which occurs at top edge portions of bearing dark banding.
That is, in the manufacture of automobile glazing for some models of automobiles, it has become popular to paint or tint the periphery of the glass, particularly rear windows and sidelights, with a dark band. In the lehr, that dark edge band absorbs heat better and tends to be hotter than the central portion of the glass sheet and may therefore sag. In addition, as is known, the top edge portion of the glass (i.e. farthest away from the mechanism which moves the glass sheet through the lehr, whether a chain drive or otherwise) tends to run hotter than the rest of the glass and as such has a tendency to droop and contact the gas heart block, resulting in marred surfaces. When the top edge portion also bears dark banding, the problem is compounded. The wider the dark banding, the greater the problem.
Accordingly, the need exists for a method and apparatus to minimize during heating in the lehr undesirable sagging of portions of the glass sheet, particularly edge portions bearing bark banding.